climax is the orgasm when you either masturbate or have sex.
for guys, its when your penis ejaculates semen or "cum" as it is widely referred to.
and im not quite sure what it is for girls.
girls, check out this http://www.puberty101.com/p_arousal.shtml
guys, check out this http://www.puberty101.com/
(p.s for guys, there is no topic for guys orgasm so just browse around)